1. Field of the Invention
The invention disclosed below relates in general to a shielded solenoid coil used in well logging. More particularly, the invention relates to a shielded induction coil useful as a sensor in a borehole logging tool and relates to the method and apparatus for shielding such coil from the capacitive effects of the borehole and other coils and apparatus of a logging tool with which the coil is associated.
2. Description of the Prior Art
Electromagnetic well logging tools have for many years employed solenoids for transmitters, receivers and other coils. Examples of such tools have included induction logging tools and electromagnetic thickness tools. The solenoidal coils are used in a number of different ways, but in general, a magnetic flux distribution is generated coaxially in the coil which interacts with the medium being measured.
For example, in an induction measurement in an open hole borehole, the magnetic field generated by alternating current in the coil creates loop or eddy currents in the surrounding formation. The receiver coil interacts with the flux associated with the eddy currents in the formation and generates a voltage signal proportional to the resistivity or conductivity of the formation.
As another example, in a electronic caliper measurement of the inside diameter of a casing, two coaxial coils are provided including a transmitter coil in which is generated an AC exciting current. A receiver coil disposed a longitudinal distance from the transmitter coil produces an induced voltage in response to the transmitter current and the impedance effect of the casing. From such exciting current and response voltage, the inside diameter of the casing may be determined.
The fundamental measurement effect of such coils is the magnetic lines of flux which are generated coaxially in the solenoidal coil. Capacitive coupling of such coils has plagued the designers of well logging equipment over the years. For example, in an induction logging environment, capacitive coupling can be detrimental to the measurement when the dieletric medium, i.e., the borehole environment, changes. In order to reduce capacitive coupling to a minimum, the prior art has provided shields about the individual coils. Such shields have included copper fingers printed on a flexible substraight. Such cylindrical shield is then wrapped about the coil.
A major shortcoming of these shields for use in induction logging is that they must be removed when balancing the transmitter, receiver and bucking coils since such coils must be physically moved in order to balance the system. The effect of these shields is small during the balancing process, but it is not negligible and cannot be balanced out easily because the shields are not present during the balancing process.
Where such solenoidal coils are used as an electronic caliper, for example, in a electromagnetic thickness tool system, the coils must be closely spaced. The prior art shields have failed to prevent capacitive coupling through the ends of the solenoid. Such coupling has in the past created problems in the design of electronic caliper coil systems in that the closer that the transmitter coil is spaced to the receiver coil, the greater the capacitive coupling has been through the ends of the coils.